Same thing with my pup (though I brought him home early) he didn't know how to play. I remember the 1st time I threw a ball and he literally just looked at the ball and then looked at me like 'Ok, what was that, mum?' Lol. It takes time for them to get it! But remember GSDs are ridiculously smart. My boy has been retrieving balls at 9-10 weeks of age. All I did was praise like a crazy person everytime he brought back the ball.
Now tug o' war. I respectfully disagree with Malibu. Tug o' war doesn't encourage aggressive behaviour, especially if he's taught from the beginning that the rope belongs to the top dog. I've taught my dog 'Let go' since day 1. I NEVER let him win (in the course of 14 months I've had him I probably let him win like 5 times when he was tiny). Everytimes he lets go, he has to sit down, and then I wait for his mouth and face expression to relax, and then I ALWAYS praise. That's how he learnt that yes, I will engage in a tug o' war if you behave, and you will get: playing time and extra affection from me. He also plays tug o' war with my 4 year-old nephew. I always stand behind my nephew so my dog can still look at me, and everytime he doesn't listen to my nephew 'Let go' I make a sound with my fingers to get his eye contact, shake my head and then he lets go. It's really good training tool to establish who's in control, in my opinion. But ONLY in a controlled environment! If tug o' warring at a dog park or daycare, that's a whole other doggo stuff.
Try and engage your boy in puzzle toys too. They love it.